The invention relates to transformers and particularly to common mode noise attenuation in transformers. While the invention has particular application to three phase transformers, we understood that the basic principles may also be applied to single phase transformers. The basic transformer has current in the primary that develops a fluctuating magnetic field. The field cuts the turns of the secondary to develop an electromotive force in the secondary. In addition to the desired electromotive force, other components that are not desired also pass over from the primary to the secondary as well as from the secondary to the primary. These undesired components are called noise. For many applications the noise is not objectionable. For many other applications the noise is objectionable and such applications include power supplies for computers and other data processing equipment, medical equipment and other voltage sensitive devices. Problems that may be encountered when such noise is transmitted may include the loss or change of data held in volatile memory or interference with electronic control equipment. For example, noise from a power line may introduce spurious signals into a computer operating system and these signals can be processed as significant data which may result in extra or missing bits which can drastically change the results. Similarly, an important factor is that certain rotating equipment, for example, may impose noise on the power line and this noise may affect other equipment that is connected to that line. Thus it is desirable to minimize both noise transferred from the primary to the secondary of the transformer, as well as from the secondary to the primary.
The prior art includes two known methods to achieve high common mode attenuation. The first involves spiral wrapping a coil in a manner similar to a "tire-wrap" using a conductive foil tape. The second uses a shield of relatively thick rigid conductor preformed by a machine into a box-like configuration which slides over the pre-insulated coil. Better attenuation is achieved by the first method because the preformed shield is arranged in closer proximity to the coil conductors. The spiral method is, however, undesirable because it is highly labor intensive. The box-like configuration is undesirable because it requires precise dimensioning and tooling, and the shield must be manufactured prior to assembly of the transformer.
The Faraday shield is well known and has been widely used. Applications include the use of a conductive foil placed between coils of the transformer to divert noise to ground. In some cases, capacitance around such a Faraday shield will still couple enough noise from the primary to the secondary to cause problems in very sensitive equipment. It is also known to use variations of the Faraday shield which is essentially a box shield which completely encloses the winding with a conductive foil. The box shield provides a ground path for primary circuit noise and has the advantage that a much smaller capacitance exists between primary and secondary coils than in the case of a simple Faraday shield.
The prior art has used various stamped metallic members which are intended to fit around at least some of the windings of a transformer. In some cases the shielding that has been employed obstructs the air flow or cooling liquid flow around the various coils in a manner that is detrimental to the life of the transformer.
The prior art includes the structures shown in the following U.S. Pat. Nos.: 2,978,658 Reaves; 3,983,522 Gearhart; 2,997,647 Gaugler et al; 4,236,133 Seiersen; 3,181,096 Raub; 4,311,977 Owen; 3,717,808 Horna; 4,454,492 Thackray; 3,886,434 Schreiner; 4,554,523 Miki et al; 3,982,814 Kaisrswerth et al; 4,571,570 Wiki et al; 3,278,877 Kameya et al; 3,560,902 Okuyama; 3,678,428 Morris et al; 3,699,488 Goodman et al; 4,042,900 Hinton et al; 4,153,891 McNutt; 4,518,941 Harada.
Of these listed Patents,.S. Pat. No. 4,042,900 Hinton et al, describes a floating electrostatic shield for disc windings. U.S. Pat. No. 3,699,488 Goodman et al, describes a static shield for each winding section which comprise a strip of aluminum-backed crepe paper. U.S. Pat. No. 4,153,891 McNutt, describes an electrostatic shield assembly for power transformer winding. Similarly, U.S. Pat. No. 4,518,941 Harada, describes two electrostatic shield foils imposed between the primary and secondary windings with an insulator disposed between the electrostatic shield foils. The other patents are only of general interest.
Some shield constructions have employed a discrete end cap for the coils, and these discrete end caps have required separate grinding wires to achieve optimum results and which thus require additional labor and materials to install.
It is an object of the invention to provide effective common-mode noise attenuation.
It is an object of the invention to provide apparatus which is inexpensive to manufacture as well as requires a minimum of labor to install.
Still another of the invention is to provide apparatus which does not obstruct cooling fluid flow (either air or oil or other fluid) adjacent to the side surfaces of the shield.
Yet another object of the invention is to provide a shield apparatus which has end caps which are integral part of the shield and thus do not require additional grounding wires for the end caps.